1. Field of the Invention
The present invention relates to a light scanning unit for scanning a beam illuminated from a light source. More particularly, the present invention relates to a light scanning unit having an improved structure capable of correcting a skew of a scanning line.
2. Description of the Related Art
Generally, the light scanning unit is adapted to be used by an apparatus, such as a laser printer, a digital copying machine, a bar-code reader, and a facsimile. A latent image is formed on a photosensitive medium through primary scanning by a beam deflector and secondary scanning by rotation of the photosensitive medium.
Referring to FIG. 1, the general light scanning unit includes: a light source 1 for generating and illuminating light. A beam deflector 7 deflects incident light so that a beam emitted from the light source 1 may be illuminated onto a surface of a photosensitive medium 15. An f-θ lens corrects an aberration included in the beam deflected by the beam deflector 7. A reflection mirror 13 directs an incident scanning line to the photosensitive medium 15 by reflecting the incident scanning line. Also, a collimating lens 3 condenses diversed light illuminated from the light source 1 to change the light into parallel light. A cylindrical lens 5 aligns this parallel light and is disposed in an optical path between the light source 1 and the beam deflector 7.
The light source 1 includes a laser diode that is on and off controlled by a control circuit. Therefore, the light source 1 illuminates a modulated beam according to an image signal. The beam deflector 7 includes a driving source 8 for providing rotational force and a polygonal mirror 9 rotatably installed on the driving source 8. The polygonal mirror 9 includes a plurality of reflection planes 9a for reflecting an incident beam. Therefore, when a single beam is illuminated onto the polygonal mirror 9, as many scanning lines are formed as the number of reflection planes 9a whenever the polygonal mirror 9 rotates one time.
A multi-beam scanning unit meets the requirements of high speed and high resolution apparatus, such as a laser printer, which uses the light scanning unit. The multi-beam scanning unit has a plurality of laser diodes as a light source, which are respectively on and off controlled independently. Multi-beams are simultaneously illuminated so that several scanning lines can be simultaneously provided on the photosensitive medium. According to the multi-beam scanning unit, as many scanning lines are formed as the number of reflection planes 9a multiplied by the number of the illuminated multi-beams whenever the polygonal mirror 9 rotates one time.
The light scanning unit or the multi-beam scanning unit having the foregoing constructions forms a line image 16 on the photosensitive medium 15 while the light source 1 is turned on and off. The photosensitive medium 15, which includes a photosensitive drum or a photosensitive belt operated to be rotatable, moves perpendicularly with respect to a scanning plane formed by the scanning lines constituting the line image 16. Therefore, the line image 16 is formed by the scanning unit and a two-dimensional image can be formed on the photosensitive medium 15 by movement of the photosensitive medium 15.
When the scanning line is formed on the photosensitive medium by the light scanning unit using a single laser diode as a light source, the photosensitive medium 15 is moved as much as a width that corresponds to the resolution while one scanning line is scanned by the beam deflector 7.
When the scanning line is formed on the photosensitive medium by the multi-beam scanning unit using a plurality of laser diodes as a light source, a plurality of scanning lines are simultaneously scanned onto the photosensitive medium by one reflection plane of the polygonal mirror. Referring to FIG. 2, presuming that lines simultaneously formed by the one reflection plane are a K-line and a line formed by the adjacent reflection plane is a K+1 line, the K-line includes simultaneously scanning lines 1, 2, . . . , n in the primary scanning direction with respect to the photosensitive medium at a predetermined interval.
The photosensitive medium 15 moves in the secondary direction (exposed-surface transferring direction) at a predetermined speed determined depending on the whole width of a plurality of scanning lines and a scanning time.
Namely, presuming that a number of laser diodes is n, a resolution is D, a rotational linear velocity of the photosensitive medium is v, a time for which one reflection plane of the polygonal mirror constituting the beam deflector rotates is t, and a movement distance of the photosensitive medium while one reflection plane of the polygonal mirror rotates is S, then S satisfies Equation 1 below.
                    S        =                              v            ×            t                    =                                                    n                ×                25.4                            D                        ⁡                          [              mm              ]                                                          (        1        )            
According to the multi-beam scanning unit having the foregoing construction, since line images by a plurality of lines (K-line) are simultaneously formed on the photosensitive medium, the line image can be formed in high speed even if the beam deflector is rotated at a low speed. Therefore, compared with the light scanning unit using a single laser diode as a light source, the multi-beam scanning unit having n laser diodes as a light source can realize the same scanning speed even if the rotational speed of the polygonal mirror having the same number of reflection planes is decreased to 1/n.
According to the multi-beam scanning unit, an exposed surface of the photosensitive medium moves a distance S while one reflection plane scans n scanning lines, as shown in FIG. 2. At this point, presuming that an effective scanning rate is ε, a distance the photosensitive medium moves while an effective scanning area is scanned is p, t1 for which the effective scanning area of the photosensitive medium is scanned is given by Equation 2 below:t1=t×ε, p=S×ε, ε<1  (2)
Here, the effective scanning rate ε is given by a ratio of a length (or a time) the effective printing area is scanned to a total length (or a time) one reflection plane of the polygonal mirror is scanned, and generally has a value of 0.5 through 0.8.
Since the photosensitive medium moves in a linear velocity v, the photosensitive medium moves a distance p for a time t1 presuming that a start point of the effective scanning area is a and its end point is b. Resultantly, the start point a and the end point b cannot be located on the same scanning plane and skew, i.e., an inclination is generated as much as p. In the meantime, as the effective scanning rate ε becomes small, the inclination of the scanning line is reduced. However, in that case, there exists a weak point that a flickering frequency of an image signal is increased.
Though a problem of skew generation in the scanning line in the multi-beam scanning unit has been described in here, such a problem still exists in the light scanning unit having a single laser diode as a light source.
Accordingly, there is a need for an improved light scanning unit having an obliquely disposed reflection mirror to correct skewed scanning lines.